Duct testing

ABSTRACT

A technique for detecting faults, blockages or holes, in a duct (6). A burst of compressed gas is injected into the duct and the decay of the pressure pulse monitored by a transducer at the gas burst launch end of the duct. A reference decay pressure that would be expected for a perfect duct is generated and the monitored decay is compared with the reference decay for deviation. A perforated duct exhibits greater pressure decay and a blocked duct less pressure decay than a perfect duct. The time occurrence of the deviation indicates the location of the fault. The signal level indicative of the pressure after a predetermined interval may also be used to measure the air friction or blowability within the duct.

FIELD OF THE INVENTION

This invention relates to detection of blockages, holes, discontinuitiesand surface qualities in ducts or tubes.

BACKGROUND OF THE INVENTION

Lightweight transmission line packages can be installed

in a duct by a procedure known as `fibre blowing`. The techniqueinvolves blowing compressed gas along the duct into which thetransmission line is to be installed and feeding the transmission linepackage into the duct at the same time so that the package is urgedalong the duct by viscous drag of the gas flow. The technique is ofparticular importance to optical fibre transmission lines which can bedamaged by the tension that is produced in pulled installationtechniques. In new installations ducts for transmission lines areusually underground or incorporated into the structure of a building andit is difficult and/or costly to install new ducting at a subsequentdate to cope with increased demand for lines or for replacement lines.Therefore when ducts are first installed additional ducts for future usewill be laid, and it may be several years after the duct installationthat fibre packages are actually blown through some of these ducts. Adegree of over capacity may also mean that there is a choice of whichduct to use. A problem that arises with this system is that, during thetime between duct laying and installation of fibre, damage may occur toone or more of the ducts which renders them temporarily or permanentlyunsuitable for use. For example, a complete blockage caused by the ductbeing crushed would make it impossible for the fibre to be installed, apartial blockage may make installation very difficult and puncture maycause such a loss of the blowing gas that installation is slow orimpossible and also exposes any installed fibre to potential damage fromwater flooding into the duct. Thus it is desirable to be able toascertain the state of a duct prior to installation, and in the event ofa fault to know its type and location. There are also occasions when thecontinuity or state of a duct containing an installed transmission linemay require monitoring.

BRIEF SUMMARY OF THE INVENTION

Accordingly the invention provides a method of testing a duct, themethod comprising introducing a pulse of compressed gas into the duct toestablish a pressure wavefront advancing through the duct, determiningafter at least one interval during travel of the wavefront along theduct a value indicative of pressure at a particular location to give anindication of a characteristic of the duct that influences the rate ofpressure change at a given location of the duct as the wavefrontadvances therethrough.

The invention also provides apparatus for testing a duct, the apparatuscomprising means for introducing a pulse of compressed gas into the ductto establish a pressure wavefront advancing through the duct, a sensorfor establishing a set of values indicative of pressure variation at aparticular location as the wavefront travels along the duct, means forcomparing said pressure variation with a set of values indicative of areference pressure variation and determining whether there is asignificant difference between the monitored and reference variationsand means for establishing the interval between the introduction of thepulse and the onset of any significant difference between the monitoredand reference pressure variations.

Another aspect of the invention provides apparatus for testing a duct,the apparatus comprising means for introducing a pulse of compressed gasinto the duct to establish a pressure wavefront advancing through theduct, a sensor for monitoring pressure at a predetermined location apredetermined interval after introduction of the pulse into the duct toestablish a value for the relative speed of travel of the wavefrontalong the duct.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is now described by way of example with reference to theaccompanying drawings in which:

FIG. 1 illustrates a prototype embodiment of the invention;

FIG. 2 shows a pressure decay trace for perfect and blocked ducts;

FIG. 3 shows a pressure decay trace for perfect and perforated ducts;

FIG. 4 shows a pressure decay trace for perfect and perforated ductswith the far end of the duct closed;

FIGS. 5 & 6 show pressure decay traces for perfect and doubly faultedducts, and

FIG. 7 is a block diagram of a preferred embodiment of the invention.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

The operation principle of the invention is the injection of a pulse ofcompressed gas into a duct which creates a pressure wavefront and thenmonitoring of the pressure decay as the wavefront travels along the ductaway from a transducer. If the duct has neither a blockage nor leakagethrough a hole then the pressure decay monitored by the transducerfollows a negative curve similar to, but not exactly, an exponentialcurve. If the duct has a blockage then the blockage impedes the pressurepulse wavefront and the pressure monitored by the transducer does notdecay as rapidly as when there is no blockage. In the event that thereis a hole in the duct then the pressure pulse is provided with anadditional escape route and so the pressure decays more rapidly. In bothcases the monitored decay of the pressure pulse follows the expectednegative curve until the pressure pulse wavefront reaches the fault, atwhich point the decay deviation to a higher or lower than expectedpressure is detected. From a knowledge of the velocity of the pulsewavefront, the time interval from the pulse launch to the onset of decaydeviation gives an indication of the distance along the duct that thefault occurs. The velocity of wavefront travel may vary between ducts(for example as a result of different levels of air friction) and is notconstant along the length of a duct and therefore it has to be generatedor predicted for the tube under test. The generation of the velocityvalue may be made by comparison with stored values for different tubesand/or from the early decay trace obtained prior to a fault togetherwith an approximation function of the velocity valuation.

A prototype test system was used to evaluate the performance of thetesting method, the apparatus being shown in FIG. 1 and comprising acompressor 1, solenoid valve 2, injection nozzle 3, pressure transducer4 and storage oscilloscope 5. The nozzle 3 and transducer assembly 4 areinserted into the end of the duct 6 to be tested. Typical ducts for usein fibre blowing are made of polyethylene with an external diameter of 8mm and an internal bore diameter of 6 mm, although ducts with externaldiameters in the range of 5 to 12 mm and internal diameters in the rangeof 3 to 10 mm are also envisaged. The ducts are fabricated in lengths of500 meters and then joined in lengths of, typically, one or twokilometers for interbuilding connections. Typical blowing installationlengths are of the order of 500 meters (or more using tandem blowing)and so an experimental length of 500 m of duct was used with closableapertures and pinch clips at intervals by means of which holes andblockages could be made. A control circuit (not shown) opens and closesthe valve in response to a push button operation to let a burst of airinto the duct. The burst of air causes an increase in pressure that issensed by the pressure transducer and input to the oscilloscope on the Yaxis against a time base X axis. Initially the duct was tested withneither blockages nor holes and a negative decay curve of pressure wasdisplayed on the oscilloscope. A blockage was then created by pinchingthe duct, a further burst of air introduced and the new pressure decaydisplayed superimposed on the trace obtained for the unblocked duct. Theresult of the superimposition is shown in FIG. 2, the blocked duct curvedeviating from the perfect duct curve after time t. This experiment wasrepeated with the duct blocked at different locations and similarresults obtained, the time taken for deviation to higher pressure beingrelated to the distance of the blockage along the duct from thetransducer. A similar set of tests was carried out for holes atdifferent locations and similar results obtained except that thedeviation observed was to a lower pressure. FIG. 3 shows a typical tracefor a perfect duct and a duct with a hole.

It was found that the superimposed early exponential decay portions,prior to deviation, and sometimes repeated traces taken under similarconditions of duct (i.e. perfect/blocked/perforated) did not always lieexactly on top of one another. This could result from a variety ofcauses including drift in the transducer output, drift in theoscilloscope amplifier, from the starting (pre air burst injection)pressure in the duct being different and also from different velocitiesof the pulse wavefront in different tubes. However, the informationrequired from the test is the shape of the trace at the deviation point,not a pressure value, and so all that is needed to be repeatable is thetime (distance) measurement of the deviation. Further, in use oninstalled ducts only the duct under test is available and it can not beblocked and unblocked as was possible in the experiment. Therefore forfield use the first portion of the decay monitored by the transducer maybe used as a trigger to generate a reference decay expected for aperfect duct and the actual decay is compared with the generatedreference. Alternatively a family of curve values may be stored and theappropriate reference curve automatically selected for comparison. Thissystem is described in more detail later.

The sensitivity of the system is improved if the deviation from thereference decay is increased, and this can be achieved by blocking offthe far end of the duct. FIG. 4 shows the effect on the decay ofblocking off the far end of a perforated duct, and from comparison withFIG. 3 it can be seen that the deviation is more noticeable. Of coursein the event of a totally blocked duct there is no benefit in blockingoff the far end. The send (transducer) end of the duct is generallyblocked so that decay in a single direction is monitored. Forcorrelation purposes or to identify blockages close to the send end, theduct may be tested from each end.

If a duct has more than one fault these may also be detected, but onlyfaults up to and including the first total blockage. Thus two holes canbe detected, the trace undergoing two deviations as shown in FIG. 5, anda hole followed by a blockage is also detectable as shown in FIG. 6.

The system may be arranged to provide a visual output of the trace,either on a chart recorder or a screen from which the installationengineer can analyse the result, or the decay and reference values maybe electronically compared and deviation in excess of a given valuelogged out as a fault with the sign of the difference value indicatingthe type of fault. Some faults such as partial blockages and multiplefaults may be easier to interpret from a trace than by electronic means.

Monitoring of pressure pulse decay also enables comparison of ducts orassessment of air friction values and may be utilised in other aspectsof quality control, and in particular for indicating `blowability` orpotential blown installation speed of ducts.

FIG. 7 shows a block diagram of a practical embodiment of the inventionfor field use. In moving from a laboratory prototype to a practicalembodiment that may be used in different temperature environments, itbecomes necessary to compensate for temperature variation otherwise thetemperature dependent drift in the circuit (especially in the pressuretransducer) can distort readings. Also, the ducts under test may varynot only in internal surface finish but the air different wavefrontvelocity: the faster the wavefront velocity the more rapid the pressuredecay, and therefore a curve from a `faster` duct compared with a`slower` duct may show a deviation that is of the same order ofmagnitude as that for a holed duct. In order to overcome this theapparatus is provided with a memory in which decay traces from a familyof ducts of varying types and at various temperatures may be stored forretrieval as references depending upon the conditions encountered. Thecircuit itself is also temperature dependent and in order to preventdrift in the pressure transducer the temperature within the circuit boxis monitored and controlled, and in the event that the temperature movesout of the controllable range an indication is given to address adifferent stored set of values for comparison, relevant to the circuitoperation at the new temperature conditions. It would be possible tohave a complete range of values for all possible temperatures or to havea wider range of temperature control, but it is preferable in terms ofsize and memory capacity to have the system described.

Referring specifically to FIG. 7, the duct 6 under test is connected toa branched connection duct 8 connected to the output of a solenoid valve7 which controls the duration of an air (or other fluid) burst into theduct. The air may be supplied from a compressed air line, generally viaa pressure regulator, or from another source and is stored in adischarge chamber connected to the solenoid. It has been foundsatisfactory to utilise a one liter discharge chamber at a pressure ofabout 2 bar. The solenoid valve operates to connect the dischargechamber to the duct for a selected period of time, intervals of theorder of 500 to 800 milliseconds having been found satisfactory. Thedischarge chamber is then recharged for the next test. It would bepossible to substitute a mechanically loaded discharge chamber (eg. achamber with a piston and compression spring held on a trigger) togenerate the air burst. With the compressed air loaded discharge chamberit has been found desirable to load the chamber via a restricted orificeof 0.5 mm in order to limit the effect of the air pressure regulatorrecharging the chamber during the discharge period.

The branch from connection duct 8 is connected to a pressure transducer9, which is preferably an a.c. bridge with its a.c. supply generatedfrom an oscillator 10. The output of the oscillator is input to anautomatic gain control 11 which adjusts the signal to a predeterminedlevel and thereby compensates for variations such as fluctuation indischarge chamber pressure or pulse duration. The signal then passesthrough a linear rectifier 12, through an automatic zeroing circuit 13which is inhibited during measurements, the inhibit auto zero signalbeing transmitted along line 14 from the start switch 15, which is alsoconnected on line 16 to trigger opening of the solenoid valve 7 andalong line 17 to the ROM 18 to activate read out of a stored perfectduct signal which is converted back into an analogue signal in thedigital to analogue converter 10 and input to a comparator 20 where itis compared with the measurement signal which is input to the comparatoron line 21. From the output of the comparator there is a feedback loop22 to the automatic gain control, the main output being input to a leveldetector 23 which determines whether the difference value signal fromthe comparator exceeds a predetermined value or `significant value`,that is a value which is greater than the error or normal variationcaused by environmental changes for example. If it does exceed thesignificant value, the signal is transmitted on line 24 to hole/blockageindicator 25 which initiates a hole or block signal for the display 26depending upon whether the difference value was positive (measuredsignal higher than reference signal) indicative of a blockage ornegative (measured signal lower than reference signal) indicative of ahole. At the same time a signal from the level detector on line 27 stopsthe clock 28 which then provides a readout of time elapsed to thedisplay; the elapsed time may be displayed in terms of distance ratherthan time. The significant value may be adjustable, particularly inresponse to temperature change in order to vary the sensitivity.

As mentioned previously, the circuit includes a temperature sensor,referenced 29, which senses the temperature in the proximity of thecircuit and activates a heater 30 to maintain the temperature at apredetermined operating temperature. It has been found convenient to setthe predetermined temperature at 35° C., which is a little higher thanthe natural temperature of the circuit environment. The temperaturesensor is also connected to the ROM 18 to change the addressed set ofreference values in the event of uncompensated temperature variation.

It has presently been found convenient to analyse ducts sections of 0 to50 meters, 50 to 300 meters and 300 to 1,000 meters, although theseranges may be altered, and it is possible to extend the ranges beyond1,000 meters. For this purpose, prior to starting the test the operatingrange is selected so that the correct range of reference curve is readout from the ROM and the comparison is delayed until the wavefront willhave reached the relevant section of duct. It is also possible. to enterother selection data such as pulse duration (which is preferablyincreased by increasing the opening time of the solenoid valve toincrease the signal level for longer ranges) and type of reference ductbest matched to the duct being tested: e.g. diameter, material type oreven a previous quality control or blowability measurement if known.

If the type of duct being tested has not been previously tested, or isan unknown type, it may be necessary to run several comparison testswith different reference duct values. This may be done manually bysystematically changing the selectable input data, or by storing themeasured data (for example in a RAM interposed between the auto zero 13and comparator 20) and then automatically repeatedly reading out themeasured data into the comparator with different references. If thereference values that are used in the comparison do not accuratelyreflect the decay curve for a perfect duct of the type being tested thenthe deviations that this poor matching causes give rise to `false alarm`faults and incorrect location of actual faults. However it is found thata poorly matched reference always indicates faults at a shorter distancefrom the launch end of the duct than the correct location. Thus whencomparing a measured decay pulse with a series of reference pulses,(whether manually or automatically) it is the reference that indicatesthe furthest distance to a fault (or in extreme instances, no fault)that is the correct reference.

It is preferable to have a display for the actual trace as well as afault location indicator, as there are some duct characteristics ordecay patterns that may be recognisable to engineers but are toosophisticated to be easily adapted to automatic diagnosis.

A modification of the system is to utilise it for determining therelative propagation velocities of an air burst within different ducts.In this technique a pulse is launched into a duct and a signal levelreading taken after a preselected period of time. On the assumption ofperfect ducts (which may for example have previously been tested forholes and blockages, and/or may have been recently fabricated and bewound on a drum) the faster the pulse travels through the tube then themore rapid is the decay, thus ducts with higher signal levels after thepredetermined interval are those in which the pulse is travelling moreslowly and which may therefore be slower to blow fibres along. If thesignal level representing pressure (e.g. voltages) is recorded forstandard air bursts (or for a normalised signal) and particular ductdimensions, then it is possible to quantify the `blowability` of ductsfor example after production and/or after installation.

We claim:
 1. A method of testing a duct having first and second ends, the method comprising the steps of:introducing at or adjacent said first end a pulse of compressed gas into the duct to establish a pressure wavefront advancing through the duct, determining after at least one time interval following said introduction and during travel of the wavefront along the duct a value indicative of pressure decay at a particular location to given an indication of a characteristic of the duct that influences the rate of pressure decay at a given location of the duct intermediate said first and second ends as the wavefront advances therethrough, whereby the location of a fault intermediate said first and second ends can be detected.
 2. A method according to claim 1 including the steps of:monitoring the pressure decay at the particular location as the wavefront advances through the duct, comparing said monitored pressure decay with a set of values indicative of a reference pressure decay to detect a significant difference between the monitored and reference pressure decays, and determining the time interval between introduction of the pulse and any significant detected difference to give an indication of the location of a fault in the duct.
 3. A method according to claim 1 in which the pressure at the particular location is established after a predetermined interval to give an indication of the average speed of travel of the wavefront along the duct.
 4. Apparatus for testing a duct, the apparatus comprising:means for introducing a pulse of compressed gas into the duct to establish a pressure wavefront advancing through the duct, a sensor for establishing a set of values indicative of pressure variation at a particular location as the wavefront travels along the duct, means for comparing said pressure variation with a set of values indicative of a reference pressure variation and determining whether there is a significant difference between the monitored and reference variations, and means for establishing the time interval between the introduction of the pulse and the onset of any significant difference between the monitored and reference pressure variations.
 5. Apparatus according to claim 4 further comprising means for varying the set of values indicative of the reference pressure variation.
 6. Apparatus according to claim 4 further comprising means for storing the set of values indicative of pressure variation and comparing such values with a plurality of different sets of values indicative of reference pressure variation.
 7. Apparatus according to claim 4 in which the temperature of the sensor is controlled.
 8. Apparatus according to claim 4 in which the pressure variation that is monitored is pressure decay as the pulse wavefront travels away from the sensor which is located proximate the end of the duct into which the pulse is introduced.
 9. Apparatus according to claim 4 in which the means for comparing comprises a monitor for displaying a trace of the sensed pressure variation.
 10. Apparatus for testing a duct, the apparatus comprising:means for introducing a pulse of compressed gas into the duct to establish a pressure wavefront advancing through the duct, a sensor for monitoring pressure at a predetermined location a predetermined time interval after introduction of the pulse into the duct to establish a value for the relative speed of travel of the wavefront along the duct.
 11. A method for detecting leak and blockages in an elongated duct, said method comprising the steps of:injecting a pulse of compressed gas into an accessible end of said duct; monitoring the resulting gas pressure decay as a function of time at a predetermined location in the duct proximate said accessible end; and comparing said gas pressure decay to a nominally perfect reference pressure decay curve (a) to detect a duct blockage if the monitored decaying pressure substantially exceeds said reference pressure decay curve, and (b) to detect a duct leakage if the monitored decaying pressure is substantially lower than said reference pressure decay curve.
 12. A method as in claim 11 further comprising the step of:determining the approximate location of a detected leak or blockage with respect to said accessible end of the duct as a function of the elapsed time interval from pulse injection until a substantial deviation from said reference pressure decay curve is first detected during said comparing step.
 13. A method for detecting a leak or blockage fault in an elongated duct, said method comprising the steps of:propagating a wavefront of compressed pressure gas along said duct from a first end toward a second end; measuring the resulting gas pressure at a predetermined fixed location in said duct during said propagating step; and comparing the measured gas pressure as a function of time with at least one reference gas pressure as a function of time, said reference gas pressure representing the expected variation of gas pressure at said location as a function of time in response to said propagating wavefront for a nominally defect-free duct and detecting a substantial deviation from the expected variation as a leak or blockage fault in the duct.
 14. A method as in claim 13 further comprising:measuring an elapsed time from a reference time until detecting said substantial deviation as a measure of the relative location of such detected fault.
 15. Apparatus for detecting leaks and blockages in an elongated duct said apparatus comprising:means for injecting a pulse of compressed gas into an accessible end of said duct; means for monitoring the resulting gas pressure decay as a function of time at a predetermined location in the duct proximate said accessible end; and means for comparing said gas pressure decay to a nominally reference pressure decay curve to detect a duct blockage if the monitored decaying pressure substantially exceeds said reference pressure decay curve and to detect a duct leakage if the monitored decaying pressure is substantially lower than said reference pressure decay curve.
 16. Apparatus as in claim 15 further comprising:means for determining the approximate location of a detected leak or blockage with respect to said accessible end of the duct as a function of the elapsed time interval from pulse injection until a substantial deviation from said reference pressure decay curve is first detected during said comparing step.
 17. Apparatus for detecting a leak or blockage fault in an elongated duct, said apparatus comprising:means for propagating a wavefront of compressed pressure gas along said duct from a first end toward a second end; means for measuring the resulting gas pressure at a predetermined fixed location in said duct during said propagating step; and means for comparing the measured gas pressure as a function of time with at least one reference gas pressure as a function of time, said reference represented the expected variation of gas pressure at said location as a function of time in response to said propagating wavefront for a nominally defect-free duct and detecting a substantial deviation from the expected variation as a leak or blockage fault in the duct.
 18. Apparatus as in claim 17 further comprising:means for measuring an elapsed time from a reference time until detecting said substantial deviation as a measure of the relative location of such detected fault. 